Apparatus for delivering electrical power to a load impedance, delivering the electrical power via a converter, generally have filters to match the power delivery of the converter to the load impedance. Typical examples of such apparatus are, for example, photovoltaic systems and wind power plants, which deliver their electrical power to a load or a load network. The filters of the apparatus allow minimisation of the power losses of the converter and are generally also used to suppress or reduce the delivery of harmonics to the load. Correct functioning of the filters is very important in matching the apparatus to the load impedance. If the filter deviates too far from the established specification, this can lead to excessive power losses of the apparatus. These can result in total failure or also destruction of the system due to poor functionality of the filter. Apparatus with high power output, in particular, for example wind turbines in the MW range, require optimum connection to the load impedances, thus for example to a network, in order to minimise power losses and failures. Also, reducing oscillations (harmonics), delivered by the converters due to the switching frequencies used, is an important function of the filter used. These harmonics can generate faults in the loads and must be sufficiently damped via the filters. In particular when delivering the electrical power to an electrical network, high demands of the power grid operator must be taken into consideration here, which call for both impedance matching and a maximum value for the harmonic oscillations (approximately 1 kHz-10 kHz). Correct functioning of each filter or group of filters, however, is important when using apparatus for delivering electrical power to a load impedance.
In the state of the art, from U.S. Pat. No. 8,898,025 B2 it is known to determine the functioning of the present filters by comparison of calculated and predefined reactive power during a transition from pre-charge to a run state of the wind turbine. In doing so sensors are used which serve to measure the power output. While the method known from the state of the art allows a check of the functioning of the filters or the mains filters of a wind turbine, the measuring transducers used are nevertheless designed for the relatively high power outputs of the wind turbine. The output currents, however, are between 10 and 100 times greater than the currents occurring in the filters or mains filters. The known method therefore allows only a very imprecise determination of the functioning of the filters. The measurement therefore does not allow precise conclusions to be drawn about specific problems of individual modules, for example due to ageing or the use of incorrect components during assembly. In addition, the checking of the filters can only take place during the system start-up procedure, meaning that a defect of the filters can only be detected during the next start-up procedure. Thus, there is a risk of damage to the wind turbine due to a defective filter or of reduced operating time through unplanned outages for repairs. This is problematic in particular in hard-to-access apparatus, for example offshore wind turbines.
Finally, European patent application EP 0 645 866 A1 discloses a method of monitoring the phase angle between filter current and filter voltage for each phase of a filter, in order to check optimum function of the filter in the range of the nominal frequency. Monitoring of the filter currents in respect of defined desired values is not known from this European patent application.
The object of the present invention is therefore to provide an apparatus for delivering electrical power and a corresponding method for operating an apparatus for delivering electrical power, allowing improved monitoring of the functioning of the filters or mains filters.